Field of the Invention
As processes for removing the sulfur oxides in exhaust gases for the purpose of preventing public nuisance there have been proposed various processes. Since the sulfur oxides are acids in nature, in order to make them harmless, after all it is necessary to neutralize them. Accordingly, most of the processes for desulfurization of exhaust gases now in operation in our country are adopting those which utilize as the neutralizing agent lime which is readily available at the lowest and most stable cost, providing gypsum as by-product. Gypsum is a harmless, incombustible, and chemically stable material, which can find many effective uses and also can be disposed of without causing any secondary public nuisance by appropriating it to land reclamation. These processes for desulfurization wherein the lime used as the neutralizing agent is recovered as gypsum, however, depend on how efficiently and steadily the sulfur oxides can be removed and how far the lime can be utilized, and therefore, how to make the process for desulfurization more feasible and economical in operation even by simplifying the steps is becoming one of the important subjects.
In the lime gypsum process, in which a slurry of calcium hydroxide, calcium oxide, or lime stone (Calcium carbonate) is used as an absorbent, the absorption reaction of the sulfur oxides is controlled by the rate of dissolution of the lime in the form of a slurry. Since in the acidic region where the solubility of lime is high the absorption efficiency of sulfur oxides is poor, and moreover, the buffer action for pH of said slurried solution is scanty, the absorption of the sulfur oxides causes a marked decrease in the pH value, adversely affecting the absorption. Thus, in order to increase the rate of absorption of sulfur oxides it is necessary either to add the lime in an amount in excess of the chemical equivalent to the sulfur oxides being absorbed so that the absorption tower can be operated in the more basic region or to resort to a multi-stage process for scrubbing. However, in the basic region scaling is liable to occur intensely in the absorption tower, etc., and also, when the pH value of the slurried solution is larger than 5, the solubility of calcium sulfite decreases forming a precipitate, so that the oxidation reaction of the calcium sulfite proceeds as a heterogeneous reaction. As a result it is necessary either to use a specially designed apparatus for oxidation or to carry out the oxidation at a pH value of the slurried solution below 4.5 after the unreacted lime in the slurry has been treated with sulfuric acid. That is to say, in the lime gypsum process the condition favorite to the absorption capacity of the sulfur oxides acts quite antagonistically to the condition required for the prevention of scaling as well as for the accelerated oxidation of the calcium sulfite. On the other hand, based on the hitherto known fact that the rate of oxidation reaction of calcium sulfite is largely affected by the pH value of the slurry, there has been proposed a lime gypsum process in which the steps have been simplified. This is a process for desulfurization in which the absorption and oxidation of the sulfur dioxide and the byproduction of gypsum are performed in a single step while contacting the slurried solution absorbent whose pH value has been made less than 5 with exhaust gases in the presence of oxygen gas. Even in such a process for desulfurization, however, the absorption capacity of sulfur dioxide is so scanty that it is applicable only to the treatment of exhaust gases whose sulfur dioxide content is as low as about 500 ppm, and moreover, on account of the poor buffer action for pH of the slurried solution it is very hard to operate the process in a steady state.